Sorting multi-dimension tables in Tableau can be a tricky business. You can set a dimension to sort descending based on the sum of sales, but it will then sort that dimension as a whole. So, if you were to sort Sub Region as follows:

It will sum up sales for each sub-region as a whole, then sort descending.

Another option is to place Sales on the row shelf and make it discrete, instead of putting it on the Text card. But, because we already have it there (for the sorting) and have hidden it, it won't appear when we add it again, so you'd need to duplicate the Sales field in order to do that.